1. Field of the Invention
The present invention relates to a measurement control device, a contour measuring instrument and a measurement control method. For example, the present invention relates to a measurement control device, a contour measuring instrument and a measurement control method used when a contour such as a profile and roughness of a surface of a workpiece is measured using a vibrating sensor.
2. Description of Related Art
There have been known contour measuring instruments that measure a contour such as a profile and roughness of a surface of a workpiece by scanning the surface of the workpiece, the contour measuring instruments including a roughness measuring instrument, a profile measuring instrument, a roundness measuring instrument and a coordinate measuring instrument.
In such measuring instruments, a vibrating force sensor (hereinafter, abbreviated as a force sensor) 1 as shown in FIG. 7 has been used, the sensor detecting a surface of a workpiece based on a minute displacement of a contact section contacting with the surface of the workpiece.
The force sensor 1 shown in FIG. 7 includes a metal base 2, a stylus 3 integrally formed on the base 2, a vibrating element 4 that vibrates (in an axial direction) the stylus 3 and a detecting element 5 that detects a vibration state of the stylus 3 and outputs the vibration state as a detection signal. A contact point (contact section) 6 formed of a diamond chip or ruby is fixedly bonded to a tip end of the stylus 3. The vibrating element 4 and the detecting element 5 are formed by one piezoelectric element, the piezoelectric element fixedly bonded on each of front and back surfaces of the base 2.
As shown in FIG. 8, when a vibration signal Pi (voltage signal) having predetermined frequency and amplitude is applied to the vibrating element 4 of the force sensor 1, the detecting element 5 obtains a detection signal Qo (voltage signal) having predetermined frequency and amplitude.
FIG. 9 shows variation in the amplitude of the detection signal Qo caused by contact with a workpiece W. In a state where the stylus 3 is not in contact with the workpiece W, when the vibration signal Pi having a certain amplitude at a resonance frequency of the stylus 3 is applied to the vibrating element 4, the stylus 3 resonates, which provides the detection signal Qo having an amplitude Ao to the detecting element 5. When the stylus 3 comes into contact with the workpiece W, the amplitude of the detection signal Qo attenuates from Ao to Ax.
A relationship between an attenuation rate k (Ax/Ao) and a measuring force is shown in FIG. 10.
Here, description will be given by taking an example of a case where the detection signal Qo in a contact state of the stylus 3 (force sensor 1) and the workpiece W is attenuated to 90% of the non-contact state (i.e., attenuation rate k=0.9). As seen from the relationship in FIG. 10, the measuring force in the contact state is 135 [μN].
Accordingly, by controlling with an actuator or the like a distance between the force sensor 1 and the workpiece W such that the attenuation rate k is always constant when the force sensor 1 contacts with the workpiece W, a profile and roughness of the workpiece W can be measured with a constant measuring force.
In the contour measuring instrument having the force sensor 1 as described above, there has been a demand for an arrangement capable of minimizing overshoot in the contact state of the force sensor and the workpiece.
Meanwhile, there have also been known contour measuring instruments having a force sensor that can perform measurement using a principle similar to that of the force sensor 1 or a principle different therefrom (see, for instance, Document 1: JP-A-2000-180156, Document 2: JP-A-2005-43177 and Document 3: JP-A-2004-77307).
There have also been known arrangements for controlling a position of a certain component (see, for instance, Document 4: JP-A-2001-166831, Document 5: JP-A-2000-89829 and Document 6: JP-A-2000-11563).
In the arrangement disclosed in Document 1, the stylus is brought into contact with a surface of the workpiece. Then, a detecting electrode detects a measuring force of the stylus and transmits a detection signal to a measuring force control circuit via a detection circuit. In the measuring force control circuit, a difference between a signal value corresponding to a preset measuring force and the signal from the detection circuit is calculated and a measuring force adjusting mechanism is controlled, thereby maintaining the measuring force between the stylus and the workpiece to a predetermined value.
In the arrangement disclosed in Document 2, a control device receives a command value from the coordinate measuring instrument, position information in X, Y and Z directions from a scale provided on a three-axis slider and an actual measuring force detected by a strain gauge. Then, an actuator is controlled by a feed-back control such that a difference between the actual measuring force detected by the strain gauge and a measuring value commanded from the coordinate measuring instrument becomes small.
In the arrangement disclosed in Document 3, after a position control is started, a Z-axis slider is brought closer to a workpiece until a predetermined time period elapses under a condition in which a pressure is maintained to a contact-judging pressure. When the predetermined time period elapses, the Z-slider is stopped. Then, an average value of the pressure during the predetermined time period is obtained, where the control of the Z-slider is switched from the position control to a pressure control when the average value reaches a target pressure.
In the arrangement disclosed in Document 4, a position command issuing section outputs, as a position command, target position data of a position to which a movable body is desired to move. A speed feed-forward pulse setting section arbitrarily sets amplitude, range and cycle of a pulse. After the position command issuing section outputs the position command, the speed feed-forward pulse setting section outputs the pulse set by the speed feed-forward pulse setting section as a speed feed-forward signal. Then, the movable body is controlled by an attenuator or the like so as to be positioned at the target position based on the speed feed-forward signal.
In the arrangement disclosed in Document 5, the overshoot is controlled to be small by a distribution mechanism that feeds back a motor-rotation-angle position signal of a motor when an absolute value of a positional deviation between a position command and a fed-back signal is large while feeds back a position signal of a machine movable section when the absolute value of the positional deviation is small.
The arrangement disclosed in Document 6 includes a coarse actuator that positions a head at a predetermined position on a disc and a fine actuator that finely adjust the position of the head that has been positioned by the coarse actuator. Then, by controlling the coarse and fine actuators, the overshoot of the position of the head is suppressed.
However, in the arrangements of Documents 1 and 2, since measuring force of the force sensor is detected and a position of the force sensor is controlled based on the detected measuring force, the overshoot in the contact state of the force sensor and the workpiece might be large, resulting in breakage of the force sensor and the workpiece.
In the arrangement of Document 3, since the entire Z-slider is controlled, the overshoot might become large due to the inertia of the Z-slider, resulting in the breakage of the force sensor and the workpiece.
In the arrangement of Document 4, since the position of the movable body is controlled based on the position command of the preset target position, it might be difficult to apply the arrangement to the contour measuring instrument in which a distance between a current position of the force sensor and the workpiece varies in accordance with a contour of the workpiece.
In the arrangement of Document 5, since a ratio of the magnitude of the fed-back signal is determined by the positional deviation between the position command and the fed-back signal, the application of the arrangement to the contour measuring instrument lowers the speed of bringing the force sensor closer to the workpiece, which might result in degradation of measuring efficiency in a measurement with a lot of measuring points.
In the arrangement of Document 6, since the position is controlled by moving the coarse and fine actuators, the control of the two actuators might be complicated when the arrangement is applied to the contour measuring instrument.